Electrochemical deoxidation of titanium
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INTRODUCTION
~N recent years, high-purity titanium has found application mainly as target material for semiconductor use. t~,2] The purity level of this titanium is between 99.999 and 99.9999 pct (excluding gaseous elements), with the major impurity being oxygen at about 0.01 pct (100 mass ppm). As the presence of oxygen in titanium leads to deterioration of titanium thin films, 131 it seems important to produce low oxygen titanium for semiconductor use. Furthermore, the development of an effective process for deoxidation of titanium could find application in the recovery of titanium scrap and in the production of low oxygen-containing titanium powders. Oxygen removal directly from titanium-oxygen solid solution to a level below 100 mass ppm is very difficult, because a large amount of oxygen dissolves in titanium to form an interstitial solid solution and because titanium has a strong affinity for oxygen. In recent years, some effective methods for removal of oxygen from titanium f4,5,6] and from titanium aluminide tT] have been developed. In these previous studies, deoxidation of solid titanium by reaction with a chemically active element, namely, calcium-saturated CaC12, was examined in the temperature range 1273 to 1473 K. The authors succeeded in producing titanium containing less than 50 mass ppm oxygen with high residual resistivity ratio (RRR = 120). t8] Calcium metal is thought to be the most effective deoxidation agent, because it has an extremely strong affinity for oxygen and because it has low solubility in solid titanium. In the calciumhalide flux deoxidation process, the flux is used to contain the deoxidation agent calcium and, in addition, to facilitate the reaction by diluting the reaction product CaO, i.e., by decreasing the activity of the by-product CaO. T.H. OKABE, Ph.D. Candidate, M. NAKAMURA, Graduate Student, T. OISHI, Associate Professor, and K. ONO, Professor, are with the Department of Metallurgy, Kyoto University, Sakyo-ku, Yoshida-Honmachi, 606 Kyoto, Japan. Manuscript submitted May 29, 1992. METALLURGICAL TRANSACTIONS B
However, there exist limitations on the impurity level of flux and the initial oxygen content of the titanium used, since the ultimate limit of deoxidation is dependent on the amount of oxygen present as an impurity in the flux and on the amount of CaO produced during deoxidation. This article reports the results of an experimental investigation directed toward the preparation of nearly oxygen-free titanium by an electrochemical deoxidation method using CaCI2 molten salt. Hashinaoto [9'1~ proposed deoxidation of titanium during electrowinning of titanium metal using fluoride molten salt baths, but contamination of titanium by oxygen or carbon was encountered. In the present study, removal of O z- (present as CaO in the flux) and production of calcium deoxidant by using an electrochemical technique are considered as means to improve the calcium-halide flux deoxidation process mentioned previously. The behavior of other contaminants, such as carbon and nitrogen, is
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